The invention features a system and method for finely tuning the rate at which ad impressions are delivered to users based on various metrics used to characterize the quality of the ad opportunities. With the present invention, more relevant ads can be selected and delivered to a wide range of recipients while still compensating for the variations in the volume of Internet traffic and usage over the course of the day. The system and method of the present invention employ a plurality of bid boundaries corresponding to different volumes of ad opportunities, thereby enabling the invention to adjust the number of ad opportunities sought over the course of day while consistently targeting the mix of ad opportunities yielding the highest response rates and margin. The bid boundaries are selected in real-time based on the behavior of prior ad recipients. The behavior may include click-through rates or entry into a geo-fence, for example.